Change advance(_back)_by to return the remainder instead of the number of processed elements
When advance_by can't advance the iterator by the number of requested elements it now returns the amount by which it couldn't be advanced instead of the amount by which it did.
This simplifies adapters like chain, flatten or cycle because the remainder doesn't have to be calculated as the difference between requested steps and completed steps anymore.
Additionally switching from `Result<(), usize>` to `Result<(), NonZeroUsize>` reduces the size of the result and makes converting from/to a usize representing the number of remaining steps cheap.
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